Membrane separation is an important unit operation in chemical processing. One known type of membrane separation utilizes reverse osmosis. In reverse osmosis, a semipermeable membrane operates as a molecular filter to separate a solution into a solute and solvent. With the solution in contact with the semipermeable membrane, an external pressure greater than the osmotic pressure of the solution is applied across the semipermeable membrane, causing the solvent to pass through, or permeate, through the membrane, while allowing the solute to remain behind.
One of the chemical processes utilizing this type of membrane separation is the manufacture of terephthalic acid. Terephthalic acid (TA) is used in the manufacture of polyesters. Polyesters in turn may be used to make fibers, films, containers, bottles, other packaging materials, molded articles, and the like.
In commercial practice, terephthalic acid has been made by liquid phase oxidation of paraxylene in an aqueous acetic acid solvent. Air or other sources of oxygen have been used as oxidants in the presence, for example, of a bromine- promoted catalyst that contains cobalt and manganese. The oxidation is exothermic and yields aromatic carboxylic acid together with by-products, including partial or intermediate oxidation products of paraxylene, such as paratoluic acid, and acetic acid reaction products (e.g., methanol, methyl acetate, and methyl bromide). Water is also generated as a by-product.
In a typical commercial process, an acetic acid-rich gaseous phase exits the reactor and enters a high pressure dehydration tower for separation of acetic acid and water. The acetic acid is recovered and returned to the oxidation reactor. A high pressure vapor phase containing small amounts of acetic acid and trace amounts of contaminants exits the tower. A portion of the high pressure vapor phase is used for energy recovery and another portion is condensed and used in other units of the process.
In order to reduce the need for fresh acetic acid solvent and for wastewater treatment, some prior processes have recovered acetic acid from the condensate. One method of recovering acetic acid from the condensate has been to use membrane separation, such as the use of a reverse osmosis membrane.
While the reverse osmosis membranes have been shown to be initially effective in recovering acetic acid, over time they have a tendency to exhibit a significant decrease in permeate flux and increase in solute passage. This decrease in performance leads to early replacement of the reverse membrane, adding a significant cost to the process.
There remains a need to efficiently and reliably recover acetic acid from an aqueous stream in chemical processes using membrane separation.